The present invention relates to a display panel or the like using organic electroluminescence (EL), inorganic EL or the like, and more specifically, to an EL display panel using an active matrix system, and the like.
Among display panels, an EL panel using organic EL or inorganic EL has received very much attention in recent years. An EL device is a chemical substance in which a material thereof emits light by being energized. The EL device can obtain a different emitted color by changing a chemical structure, and a research on utilizing the EL device for a display panel has been progressed. In the EL device, the organic EL, hereinafter, referred to as an OLED (organic light emitting diode), is one emitting light in a manner that a direct current is flown through a fluorescent organic compound excited by application of an electric field. The organic EL has received attention as a next-generation display device in terms of its thin thickness, wide viewing angle, wide gamut and the like. In a drive method of this OLED, a passive type and an active type are present, and in order to realize a large screen and high definition display, the active type is suitable in terms of material, lifetime and crosstalk. In this active type, generally, a TFT (thin film transistor) drive method is adopted.
For a pixel circuit of an active matrix OLED (AMOLED), a variety of configurations have been proposed in papers and the like. FIGS. 23(a) to 23(d) are diagrams showing examples of a variety of pixel circuits for the OLED. FIG. 23(a) shows a voltage programming circuit composed of two TFTs, which is proposed by Brody [Brody et al, “A 6×6 inch 20 lpi Electroluminescent Display Panel”, IEEE Trans. Elec. Dev., Vol. ED-22, pp. 739–748 (1975)]. This is a mainstream configuration at present. FIG. 23(b) shows a voltage programming circuit composed of four TFTs, which is added with a Vth compensation function and proposed by Dawson [Dawson et al., “A Poly-Si Active-Matrix OLED Display with Integrated Drivers,” Digest of Technical Papers Volume XXX, 1999 SID International Symposium, pp. 438–441; “Pursuit of Active Matrix Organic Light Emitting Diode Displays,” Digest of Technical Papers Volume XXXII, 2001 SID International Symposium, pp. 372–375]. FIG. 23(c) shows another current programming circuit composed of four TFTs, which is added with the Vth compensation function and proposed by Dawson. Moreover, FIG. 23(d) shows current programming circuit composed of four TFTs of a current mirror system. In each of the variety of pixel circuits shown in FIGS. 23(a) to 23(d), a drive TFT denoted by a reference symbol T2 (p) is used, and an OLED is connected thereto. Moreover, a power source is supplied to each pixel circuit through common wiring denoted by a reference symbol Vdd.
As a background art described in a gazette, for example, there is a technology in which switch means is inserted between a light-emitting device and current supply means in order to reduce power consumption of a display panel using current control-type light-emitting device represented by the organic EL device and the like, e.g., refer to Gazette of Japanese Patent Laid-Open No. 2002-278497 (p. 4, FIG. 3). Moreover, in order to prevent an occurrence of a residual image in the organic EL device, for example, there is a technology in which a TFT for connecting a potential of a pixel electrode to a negative power source and a TFT for resetting a pixel capacity are added, thus making it possible to turn off the organic EL device before programming, e.g., refer to Gazette of Japanese Patent Laid-Open No. 2002-244617 (pp. 3 to 4, FIG. 1). Furthermore, for example, in a liquid crystal display panel and the like, there is disclosed a technology of inspecting lighting of pixels in a manner that common wiring is separated into two lines, short-circuit wiring short-circuiting each of the common wiring and short-circuit wiring short-circuiting all of segment wiring are formed in advance, and a drive voltage is applied to each pixel through each of these pieces of wiring, e.g., refer to Gazette of Japanese Patent Laid-Open No. 2002-328627 (p. 3, FIG. 1).
Here, a manufacturing process of the active matrix OLED (AMOLED) is broadly divided into an array step of creating a TFT array and a cell step of forming the OLED on the array. If the TFT array can be inspected after the array step, a defective TFT array can be prevented from flowing out to the cell step, and a reduction of manufacturing cost can be expected. In the pixel circuit of the AMOLED, as shown in FIGS. 23(a) to 23(d), there is a drive TFT (T2(p) shown in FIGS. 23(a) to (d)) for driving the OLED, and characteristic variations of this drive TFT give serious and negative effects on image quality of an AMOLED panel. Accordingly, it is desired to perform a characteristic inspection of this drive TFT before forming the OLED. However, since the OLED is not connected to the drive TFT in the pixel circuit before forming of the OLED and the drive TFT becomes an open drain or an open source, a current path is not present in the drive TFT. Accordingly, a current cannot be flown to the drive TFT, and the characteristic inspection is also very difficult.
In each of JP2002-278497 and JP2002-244617 described above, the TFT is added to the pixel circuit, and thus an additional function other than an original image display function is realized. However, in the technology described in JP2002-278497 described above, only a mechanism of controlling emission duty in display is realized, and this cannot be utilized for the inspection of the drive TFT. Moreover, in the technology described in JP2002-244617 described above, the drive TFT is forcibly fixed to an OFF state when the proposed function is used, and this cannot be applied to the purpose of the inspection. Furthermore, although the technology described in JP2002-328627 described above has a point similar to a configuration of the present invention to be described later in that the common wiring is separated into two lines, the technology does not have any related technical portion other than the above, and cannot be utilized for the inspection of the drive TFT for driving the EL device.